The present invention relates generally to a method for optimizing the coefficient of performance of a transcritical vapor compression system by measuring the heat sink inlet temperature and adjusting the high side pressure to an optimum value according to a preset control strategy.
Chlorine containing refrigerants have been phased out in most of the world due to their ozone destroying potential. Hydrofluoro carbons (HFCs) have been used as replacement refrigerants, but these refrigerants still have high global warming potential. xe2x80x9cNaturalxe2x80x9d refrigerants, such as carbon dioxide and propane, have been proposed as replacement fluids. Unfortunately, there are problems with the use of many of these fluids as well. Carbon dioxide has a low critical point, which causes most air conditioning systems utilizing carbon dioxide to run transcritical, or above the critical point.
When a vapor compression system runs transcritical, the refrigerant does not change phases from vapor to liquid while passing through the heat rejecting heat exchanger. Therefore, the heat rejecting heat exchanger operates as a gas cooler in a transcritical cycle, rather than as a condenser. The pressure of a subcritical fluid is a function of temperature under saturated conditions (where both liquid and vapor are present). However, the pressure of a transcritical fluid is a function of fluid density when the temperature is higher than the critical temperature.
It is important to regulate the high side pressure of a transcritical vapor compression system as the high side pressure has a large effect on the capacity and efficiency of the system. In one prior system, the optimal coefficient of performance is maintained by sampling the refrigerant temperature and pressure at the outlet of the gas cooler and adjusting the high side pressure to an optimum value according to a predetermined control strategy. In another prior system, the high side pressure and low side pressure are coupled based on a pre-determined control strategy to adjust the high side pressure to an optimum value to maintain the optimal coefficient of performance.
A transcritical vapor compression system includes at least a compressor, a heat rejecting heat exchanger, an expansion device, and a heat accepting heat exchanger. Of course, this is a simplified system and other components may be included. Refrigerant circulates through the closed circuit system. Preferably, carbon dioxide is employed as the refrigerant. High pressure refrigerant flowing through the heat rejecting heat exchanger is cooled by a fluid, such as water, flowing in an opposing direction through a heat sink. The vapor compression system further includes a heat pump to reverse the flow of the refrigerant and change the system between a heating mode and a cooling mode.
In a transcritical vapor compression system, the high side pressure is independent of the operating conditions. Therefore, for any set of operating conditions, it is possible to operate the cycle at a wide range of high side pressures. For any set of operating conditions, there is also an optimal high side pressure which corresponds to an optimum coefficient of performance. Two variables determine the operating conditions: the outdoor air temperature and the heat sink inlet temperature. As the outdoor air temperature only slightly influences the optimal high side pressure, and therefore the coefficient of performance, only the heat sink inlet temperature significantly affects the optimal high side pressure.
In selecting the optimal high side pressure, and therefore achieving the optimal coefficient of performance, a temperature sensor measures the heat sink inlet temperature. For any heat sink inlet temperature, a single optimal high side pressure is selected to optimize the coefficient of performance. The optimal high side pressure for each heat sink inlet temperature is preset into a control and is based on data obtained by previous testing. A pressure sensor continually measures the high side pressure. If the high side pressure is not optimal, the expansion device is adjusted to alter the high side pressure to the optimal value.